In the field of radiography, it is common to enclose an unexposed x-ray film in a light-tight cassette, which allows the radiographic procedure to be conducted in normal room lighting. The cassette is normally made from materials such as plastics, aluminum, glass fiber composites, or carbon fiber composites, which are opaque to visible light, but relatively transparent to x-rays. Since x-ray films are not highly efficient at producing latent images from x-ray exposure, it is common to include image intensifying screens within the cassettes to increase imaging efficiency.
Typically, two intensifying screens are mounted on resilient foam pads, which are in turn, mounted on the opposing inner faces of the x-ray cassette. The x-ray film is placed between the opposing intensifying screens, which are urged into intimate contact with the film by the resilient foam pads. A latching mechanism and hinge allow the cassette to be opened so that exposed films can be removed for processing and unexposed films can be loaded into the cassette.
Whereas the use of intensifying screens significantly decreases the amount of x-ray energy required to create a usable image on the film, there is an inherent loss in image sharpness as compared to an image formed without the aid of intensifying screens.
An intensifying screen, typically made with rare earth phosphors, absorbs energy from the x-ray beam, and then re-emits the energy in the visible portion of the electromagnetic spectrum. The emitted visible wavelengths, principally blues, expose adjacent areas of the x-ray film.
Since these visible wavelength emissions are omni-directional, any separation between the emitting phosphor and the light-sensitive layers of the film result in the image of each discrete point on the screen being formed over a finite area on the film. In a practical sense, the image of the edge of an object being x-rayed should result in a stepwise increase in the density of the image on the processed film, however, because of the spread of imaging light from the intensifying screen at the imaged edge, the resulting density increase occurs over a finite distance, thus the image of the edge is not sharp.
In conventional radiography, both sides of the film are each coated with a light-sensitive layer and a clear, protective overcoat layer. Intensifying screens are coated with a rare-earth phosphor layer and clear protective overcoat layer. Some loss in image sharpness results from the spread of light emitted from the phosphor layer as it passes through the overcoat layers. Some of the light emitted by each intensifying screen passes through the light sensitive layers adjacent the screen, and through the film base, exposing the light-sensitive layer on the opposite side of the film. Since the opposite side light-sensitive layer is separated from the emitting phosphor layer by a significant distance, this secondary portion of the image is poorly resolved. For general radiography, the size of the object of interest obviates the need for maximum image sharpness, and the desire to minimize the patient exposure to x-rays dictates the use of the two-screen system.
In the specialized field of mammography several unique problems emerge. The size of the objects of interest, small calcifications and fibrils in breast tissue, dictate the use of a system with superior image detail forming capability. Differentiation of calcifications and fibrils within normal breast tissues requires the use of x-rays with low penetrating power (longer wavelengths) and further dictates a superior imaging system to prevent the loss of image contrast which would degrade such differentiation. The need to image the entire breast tissue mass requires that one edge of the film be capable of being located very close to the patients chest wall.
In conventional mammography it is common to utilize a light-tight cassette generally made from a plastics material and specifically designed to contain the film with at least one edge of the film adjacent to an exterior side wall of minimum thickness. Although some two-screen mammography systems are used, the more common practice is to utilize a film with light-sensitive layers coated only on one side, and a single screen urged into intimate contact with the light-sensitive side of the film. Typically, this intensifying screen is mounted on the surface of a resilient foam pad which is mounted to the cover of the cassette. The cover is attached to the cassette bottom by means of a hinge and provided with latching means to hold the cassette in the closed condition. The foam pad is sized in thickness such that it is partially compressed when the cassette is closed, thus urging the screen into contact with the film.
In conventional mammography cassettes, several problems arise which contribute to losses in image sharpness resulting from poor film/screen contact.
In the typical mammography cassette, the cassette bottom and the cassette cover are essentially flat planar surfaces, and the typical resilient foam pad is uniform in thickness. In use, the film is placed into the bottom of the cassette and the cover is rotated on its hinge to the closed position. The cover mounted intensifying screen essentially approaches the film at right angles to the film surface making contact with the film over essentially the entire film area often trapping pockets of air between the film and the intensifying screen, thus preventing intimate film/screen contact over some significant areas.
Screens are typically manufactured by coating a layer of rare-earth phosphor in a binder, onto a flexible plastic base, overcoating the phosphor layer for physical protection, and balancing the curl tendency of the above layers by coating curl control layers on the back side of the plastic base material. Small anomalies in the flatness of the surface result from thickness imperfections in the base material, coated layer thickness variations, and/or variability in the drying of the coated layers which induces local variability in the curl tendency of the layers.
In the typical mammography cassette manufactured from a plastics material generally by the injection molding process, imperfections in the mold surface, variability in the injection molding process temperatures and pressures, variability in the plastics material itself, and non-uniformity in the cooling of the part within the mold may result in local disturbances in the flatness of the cassette surface against which the film is loaded.
Also, in a typical mammography cassette, the flat design of the cassette cover and body limits the density of the foam pad and the amount of compression that can be tolerated. Typical internal pressures are approximately 0.1 psi or less, and evaluation of these cassettes has shown that this low level of pressure is insufficient to overcome local anomalies in the surface flatness of screens and cassettes, and local areas of poor contact exist, with the diameter of these areas typically 0.5 inches, and with the separation of the film and the screen typically 0.0005 inches at the center of the area. This degree of separation is sufficient to significantly degrade the sharpness of the image in the affected area.
Attempts to incorporate denser and/or thicker foam pads to increase the film/screen contact pressure have proven unsuccessful because the higher internal pressure produces an unacceptable degree of bulging of the cassette. Bulging of the cassette cover and body panels results in uneven contact pressure and may cause a large area of non-contact at the center of the cassette. Bulging of the cassette may also render it unusable in the cassette holders typically incorporated into mammography x-ray apparatus.
The present invention provides an improved mammography cassette which minimizes and/or eliminates the problems experienced in prior art cassettes. The improved cassette features a cassette body and a pressure plate which are designed to develop a uniform pressure over the entire area of film/screen contact at a substantially higher pressure than current cassettes. The screen is adhesively laminated to the smooth rigid surface of the pressure plate, thus correcting local screen surface anomalies resulting from non-uniform curl and drying. A resilient foam pad is interposed between the film and the cassette bottom effectively preventing any imperfections in the bottom surface from affecting the film/screen contact and urging the film into intimate contact with the intensifying screen. A cover carries one portion of the cassette latching means and is attached to the cassette bottom by a plastic living hinge. A window is provided for the optical imaging of patient identification onto the film enclosed in the cassette.
The proper shape for the pressure plate and cassette body are determined by a finite element modeling technique. Initially, the shapes are assumed to be flat parallel surfaces. When the appropriate loads are applied, constant pressure on the film/screen areas, and uniform loads along suspension spring surfaces, the load deformation can be determined from an appropriate finite element model. The predicted deformation represents the error from the desired flat parallel state. The original model is then corrected and the resulting model is again loaded and its deformation determined and again compared to the desired state. This iterative process is continued until the predicted error is within acceptable limits.